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May 10, 2006
From: Leanne & David Atkinson

Dear Bill,
Thought this behaviour might be of interest. We found Petalifera ramosa on a soft green weed that is called locally sea lettuce. When we moved the weed to get a clearer photo of the Petalifera ramosa it withdrew the filamentous branches and terminal papillae into the tubercles just as some nudibranchs withdraw their gills into a gill pocket.

See photos attached of the same Petalifera ramosa before being disturbed and while the sea lettuce is being held in place by a black gloved finger in the bottom left corner, for the photograph. What is the purpose of the filamentous branches are they gills?

Dear Leanne & David,
Thanks for these photos which show the behaviour of the papillae so clearly. Although I don't think anyone has proved the papillae are secondary gills, the fact that they are filled with blood, and thin-walled, would suggest they are involved in gas exchange like a true gill. The body wall in most Sea Hares is quite thick and muscular so its unlikely that gas exchange can take place directly through the skin except in specialised 'thin' places like the papillae.

Many opisthobranchs have papillae and branching structures which are probably used as secondary gills. A secondary use for these structures is that they help camouflage the animal by breaking up its outline or silhouette and so make it less visible to likely predators. For example Tritoniopsis elegans looks spectacular and very obvious when photographed on a black background [see Fact Sheet] but in its natural environment it is almost invisible.
Best wishes,
Bill Rudman